The present invention relates to a control circuit and method for a piezoelectric transformer and, more particularly, to a control circuit and method for a piezoelectric transformer suited for use in a driving apparatus for a cold-cathode fluorescent lamp.
Recently, liquid crystal displays are extensively used as display devices of, e.g., portable notebook personal computers. These liquid crystal display devices incorporate a cold-cathode fluorescent lamp as a so-called back light in order to illuminate a liquid crystal display panel from the back. Turning on this cold-cathode fluorescent lamp requires an inverter capable of converting a low DC voltage of a battery or the like into a high AC voltage of 1,000 Vrms or more in an initial lighting state and about 500 Vrms in a steady lighting state. Conventionally, a winding transformer is used as a boosting transformer of this inverter. In recent years, however, a piezoelectric transformer which performs electric conversion via mechanical energy and thereby performs boosting is beginning to be used. This piezoelectric transformer has a generally unpreferable characteristic, i.e., largely changes its boosting ratio in accordance with the magnitude of an output load (load resistance) such that the boosting ratio is high for a small output load (large load resistance) and is low for a large output load (small load resistance). On the other hand, this dependence upon a load resistance is suited to the characteristics of an inverter power supply for a cold-cathode fluorescent lamp. Accordingly, a piezoelectric transformer has attracted attention as a small-sized, high-voltage power supply meeting the demands for a low profile and a high efficiency of a liquid crystal display. An example of a control circuit for this piezoelectric transformer will be described below with reference to FIG. 1.
FIG. 1 is a block diagram of a piezoelectric transformer control circuit as the prior art.
In FIG. 1, reference numeral 101 denotes a piezoelectric transformer; 102, a load such as a cold-cathode fluorescent lamp connected to the output terminal of the piezoelectric transformer 101; 103, a detecting resistor Rdet for detecting a current flowing in the load; 104, a rectifying circuit for converting an AC voltage generated in the detecting resistor 103 into a DC voltage; 105, an error amplifier for comparing a voltage Vri rectified by the rectifying circuit 104 with a reference voltage Vref and amplifying the difference as a comparison result; 106, a voltage-controlled oscillation circuit for outputting an oscillation signal in accordance with the output voltage from the error amplifier 105; and 107, a driving circuit for driving the piezoelectric transformer 101 in accordance with the oscillation signal from the voltage-controlled oscillation circuit 106. The operation of the control circuit with the above configuration will be described below with reference to FIGS. 2A and 2B.
FIGS. 2A and 2B are graphs for explaining an example of frequency characteristics for the output voltage and load current of a piezoelectric transformer.
As shown in FIG. 2A, the piezoelectric transformer 101 has a hilly resonance frequency characteristic whose peak is the resonance frequency of the piezoelectric transformer 101. It is generally known that a current flowing in the load 102 due to the output voltage from the piezoelectric transformer 101 also has a similar hilly characteristic. In FIG. 2B, this load current is represented by a load current detection voltage Vri. Control using a right-side (falling) portion in this characteristic will be described below. When the power supply of this control circuit is turned on, the voltage-controlled oscillation circuit 106 starts oscillating at an initial frequency fa. Since no current flows in the load 102 at that time, the voltage generated in the detecting resistor (Rdet) 103 is zero. Accordingly, the error amplifier 105 outputs a negative voltage, as a result of comparison of the load current detection voltage Vri with the reference voltage Vref, to the voltage-controlled oscillation circuit 106. In accordance with this voltage, the voltage-controlled oscillation circuit 106 shifts the oscillation frequency of an oscillation signal to a lower frequency. Therefore, as the frequency is shifted to a lower frequency, the output voltage from the piezoelectric transformer 101 rises, and the load current (load current detection voltage Vri) also increases. When the load current (load current detection voltage Vri) become equal to the reference voltage Vref, the frequency stabilizes (fb). Even if the resonance frequency changes due to a temperature change or a change with time, the frequency shifts accordingly to always hold the load current substantially constant.
In the control circuit shown in FIG. 1, therefore, frequency control is so performed that the load current detection voltage Vri becomes equal to the reference voltage Vref, and the load current is held at a predetermined value by this frequency control. When a cold-cathode fluorescent lamp is used as a load in this piezoelectric transformer control circuit and the control circuit is used as a lighting device for the cold-cathode fluorescent lamp, a function of holding the luminance of the cold-cathode fluorescent lamp at a predetermined luminance can be achieved since the luminance of the cold-cathode fluorescent lamp is proportional to a lamp current flowing in the cold-cathode fluorescent lamp.
In this conventional piezoelectric transformer control circuit, however, when the load connected to the output terminal of the piezoelectric transformer is disconnected due to some reason, and the output terminal changes to a so-called open state, a high voltage is generated at the output terminal of the piezoelectric transformer in accordance with the open state.
At this time, if the frequency of the oscillation signal for driving the piezoelectric transformer does not vary, such a high voltage as to induce physical damage to the piezoelectric transformer is not generated because the operating point of the piezoelectric transformer shifts from the resonance frequency. However, when the output terminal of the piezoelectric transformer 101 actually changes to the open state, for example, the above control circuit shown in FIG. 1 detects that no current flows through the load current detecting resistor (Rdet) 103, and the oscillation frequency (operating point) of the voltage-controlled oscillation circuit 106 is swept to a lower frequency by the function of keeping the load current substantially constant. As a result, the operating point of the piezoelectric transformer 101 becomes equal to the resonance frequency to generate such a high voltage (10 kV or higher) as to physically damage the piezoelectric transformer 101 at the output terminal.
In the control circuit of FIG. 1, assume that the output of the piezoelectric transformer 101 is short-circuited to, e.g., ground (GND) (so-called grounding) due to some reason (note that the output may be short-circuited to a negative potential). Even in this state, if the frequency of the oscillation signal for driving the piezoelectric transformer 101 does not vary, no physical damage to the piezoelectric transformer 101 is induced. In practice, however, similar to the open state, when the control circuit detects that no current flows through the load current detecting resistor (Rdet) 103, the operating point of the piezoelectric transformer 101 is swept to a lower frequency by the voltage-controlled oscillation circuit 106 with the function of keeping the load current substantially constant, and the operating point of the piezoelectric transformer 101 may pass a resonance point (near the peaks in FIGS. 2A and 2B). Since the piezoelectric transformer 101 is in a series resonance state at a resonance frequency unique to the piezoelectric transformer, and the output of the piezoelectric transformer 101 is short-circuited to GND, the input impedance of the piezoelectric transformer 101 becomes very small. Accordingly, a large current may flow through the piezoelectric transformer 101 to physically damage it.
For example, Japanese Patent Publication No. 59-40313 and Japanese Patent Laid-Open No. 5-64436 disclose the technique of detecting the load current using a resistor to detect an increase in current caused by a short circuit. In these references, however, the short-circuiting state means a short circuit between the output terminals of a piezoelectric transformer connected to a load, and a short circuit between the output of the piezoelectric transformer and GND cannot be detected. In the control circuit disclosed in these references, when a short circuit occurs between the output terminals of the piezoelectric transformer connected to the load, the load current is detected, and the frequency is shifted to a higher frequency to prevent damage to the piezoelectric transformer.
It is an object of the present invention to provide a control circuit and method for a piezoelectric transformer in which damage to the piezoelectric transformer can be prevented upon occurrence of an abnormal state at the output of the piezoelectric transformer.
To achieve the above object, a piezoelectric transformer control circuit of the present invention has the following arrangement.
That is, a piezoelectric transformer control circuit having oscillation means for generating an oscillation signal in accordance with a control voltage, driving means for driving a piezoelectric transformer by an AC voltage generated in accordance with the oscillation signal from the oscillation means, and control means for detecting a load current of a load connected to an output side of the piezoelectric transformer and controlling an oscillation frequency of the oscillation means so as to keep the load current substantially constant, is characterized by comprising protective means for detecting an output voltage from the piezoelectric transformer and protecting the piezoelectric transformer on the basis of the output voltage.
For example, the protective means is characterized by comprising detecting means for detecting an open state or short-circuiting to ground on the output side of the piezoelectric transformer in accordance with a result of comparing the detected output voltage with a predetermined value. The protective means preferably stops generation of the oscillation signal by the oscillation means or sweeps a frequency of the oscillation signal by the oscillation means to a predetermined frequency so as to protect the piezoelectric transformer when the detecting means detects the open state or the short-circuiting to ground.
To achieve the above object, a piezoelectric transformer control method of the present invention has the following steps.
That is, a piezoelectric transformer control method of generating an oscillation signal in accordance with a control voltage, driving a piezoelectric transformer by an AC voltage generated in accordance with the oscillation signal, detecting a load current of a load connected to an output side of the piezoelectric transformer, and controlling an oscillation frequency so as to keep the load current substantially constant, is characterized by comprising the steps of detecting an output voltage from the piezoelectric transformer, and detecting an open state or short-circuiting to ground on the output side of the piezoelectric transformer in accordance with a result of comparing the detected output voltage with a predetermined value.
The method preferably further comprises stopping generation of the oscillation signal or sweeping a frequency of the oscillation signal to a predetermined frequency so as to protect the piezoelectric transformer when the open state or the short-circuiting to the ground is detected.
With the above arrangement and steps, damage to the piezoelectric transformer can be prevented upon occurrence of the open state or the short-circuiting to ground on the output side of the piezoelectric transformer.
Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof.